DTT vacuum vessel, from mechanical design to fabrication requirements

DTT is one of the largest superconducting tokamaks with the mission to get scientific and technological proofs of power exhaust in prospect of the first nuclear fusion power plant. The 5.5MA maximum plasma current, 6T toroidal magnetic field at the plasma center, and 2.19m plasma radius make DTT a flexible and compact facility for testing D-shaped plasmas with different divertor configurations for heat load spreading. The DTT experimental campaigns will take place inside the vacuum vessel, a hermetically sealed stainless steel container that houses the fusion plasma, acts as a first containment barrier for radioactivity, and provides supports for in-vessel components (first wall, divertor, in-vessel coils, stabilizing plates). Eighty-two ports in the vacuum vessel provide penetration access for remote handling operations, diagnostics, plasma heating, and vacuum system services. The main part of the vacuum vessel is the vacuum chamber which will measure about 7 m outer diameter, 3.9 metres high, and weigh approximately 45 ton. The vacuum chamber is made of an inner shell and an outer shell realising a double-walled structure circulated in the inter-shell by a fluid, i.e. borated water during advanced experimental campaigns as shielding media and as mitigation solution to guarantee sufficient protection of the superconducting coils and to reduce the neutron streaming and the neutron-induced radioactivity. The inter-shell ribs have holes ensuring that the fluid flows in all the parts of the inter-shell. Being the inter-shell pressurised up to 4 bar, the vacuum chamber is also a pressure vessel. All longitudinal and circumferential welds within the main shells will be full penetration butt joints. Reinforcing ribs are positioned in the inter-shell as nonpressure stiffeners, moreover support pads are integrated in the innershell as permanent attachments of the in-vessel components. The design of the vacuum vessel, as those of the invessel components, is addressed mainly by taking disruptions as prevailing loads for structural integrity verifications, whereas the local seismic response is important for the integration of all the tokamak systems. Port stubs are integrated with the inter-shell as nozzles. Reinforcing ribs and port stubs are also attached through full penetration welds. These types of welds are required also for improving (a) the vacuum compatibility and the leak testing with vacuum sealing joints, (b) the corrosion resistance of the rib joints in the inter-shell. The type and the extent of the nondestructive examinations (NDE), in particular 100% volumetric, allow to maximise the weld joint efficiency at the inner shell, whereas 10% volumetric inspection can be tolerated at the outer shell simplifying the fabrication. Other NDE will be performed on the procured raw material (volumetric) and on parts during manufacturing (visual, helium leak, magnetic permeability, dye penetrant at the back side e.g. excluding the plasma side of the torus vacuum boundary). All these topics, from the design to the fabrication requirements, will be explained with this work.